Thank you for your question, my name is Steve and I am a fully qualified vehicle technician with over thirty years in the trade, hopefully I can help.
The engine management light is connected to the ECU and will flag up any problem that is found in the management system. To fully access the problem the vehicle will need to be connected to a diagnostic scanner to read off the fault codes, once these are obtained a more accurate diagnosis can be made.
If your local garage has read off the codes then this may help determine the problem, if they did not then the codes these do need to be read and I would advise using a garage who have the Mercedes Star diagnostic system.
Your garage seem to be giving very vague information and they should supply you with more facts as to the reason for the price they have given.
I hope this answers your question but do please feel free to ask any further details by replying to this thread.
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